A number of approaches have been used to encapsulate bioactive agents into microspheres of polymers for sustained release. Most of them are based on phase separation (U.S. Pat. No. 4,673,595, EP 52,510), cryopulverization after melt extrusion (U.S. Pat. Nos. 5,134,122, 5,192,741, 5,225,205, 5,431,348, 5,439,688, 5,445,832 and 5,776,885), double emulsion evaporation (w/o/w, water/oil/water) (U.S. Pat. Nos. 4,652,441, 4,711,782, 4,954,298, 5,061,492, 5,330,767, 5,476,663, 5,480,656, 5,611,971, 5,631,020 and 5,631,021), single emulsion evaporation (o/w, oil/water) (U.S. Pat. Nos. 4,389,330 and 5,945,126; Shameem M, Lee Hee Yong, DeLuca P. P., AAPS Pharmsci., 1 (3) article 7, 1999; Kostanski J. W., Pharm. Dev. Tech. 5, 585-596, 2000), and spray drying (IE920956).
Phase separation method is a process for preparing microspheres in which a biodegradable polymer is dissolved in an excessive amount of an organic solvent, such as methylene chloride, and a drug dissolved in a small amount of water is added to the polymer solution with stirring. Silicon oil is then added to the polymer-drug mixture at a constant rate to form embryonic microspheres, and an excessive amount of a non-solvent, such as trichlorofluoromethane, is added to the solution to extract the organic solvent from the embryonic microspheres. The solidified microspheres are recovered by filtration, and dried under pressure. However, the phase separation method is problematic as follows. Since the toxic solvent such as methylenechloride is not sufficiently removed by drying under pressure, the residual solvent reduces the stability of formulations, and may also be detrimental to health when administered to the body. Also, the excessive use of non-solvent, such as freon, hexane, heptane, cyclohexane, and trichlorofluoromethane, for the solidification of embryonic microspheres is not cost-effective upon mass production and causes serious environmental contamination.
By contrast, cryopulverization after melt extrusion permits minimal use of toxic solvents. This method is a process for preparing microspheres in which a mixture of a biodegradable polymer and a drug is melt-extruded through an extruder at a high temperature and pulverized at a low temperature. The biodegradable polymer-drug mixture may be obtained by homogeneously mixing a polymer and a drug in a solvent, such as methylene chloride, with an agitator, and removing the organic solvent using a rotary evaporator or a vacuum dryer, or by cryo-milling at a low temperature and sieving each powder and blending the two fine powders. The latter case does not have the problem of the residual toxic solvent because it does not employ a toxic solvent during microsphere preparation. However, the procedure for preparing microparticles does not exclude the possibility of an interaction between the polymer and the drug and denaturation of the drug due to high temperature and high pressure upon melt-extrusion, and denaturation of the drug due to heat locally generated during cryopulverization. This method is also difficult to use to make microspheres having a uniform size, which are thus easy to inject.
The two methods for preparing microspheres, in addition to the problems of residual solvent, difficulty in mass production and drug denaturation, have another disadvantage in that a biodegradable polymer used for the sustainable release of a drug is non-hydrophilic and thus poorly dispersible in an aqueous suspension for injection.
Water-in-oil-in-water (w/o/w) double emulsion evaporation has commonly been applied to encapsulate hydrophilic drugs, such as peptides or proteins, into polymeric microspheres. In this W/O/W method, a hydrophilic drug is dissolved in water, and this aqueous phase is dispersed in an organic phase containing a biodegradable polymer to yield a primary emulsion (water in oil). This primary emulsion is again dispersed in a secondary aqueous phase containing an emulsifier. Single emulsion evaporation (oil in water (o/w)) has been commonly used in the encapsulation of lipophilic drugs. In this O/W method, a drug and a biodegradable polymer are co-dissolved in a mixture of suitable organic solvents (e.g., methanol and methylene chloride), and the resulting solution is dispersed in an aqueous phase. In both emulsion evaporation methods, as an organic solvent is removed by extraction or evaporation during polymer dispersion in an aqueous phase, the polymer decreases in solubility and is thus solidified to form microspheres. In these methods, the technically important factor is the encapsulation efficiency of bioactive drugs.
Most hydrophilic drugs leak in large amounts when dispersed in an aqueous phase, resulting in low encapsulation efficiency. To solve this problem, Okada et al. employed material such as gelatin in the microsphere preparation based on double emulsion evaporation. This material increased the viscosity of a primary emulsion and decreased the diffusion rate of a drug (an LHRH derivative) into a secondary emulsion, resulting in enhanced drug encapsulation (Okada, H. and Toguchi, H., Crit. Rev. Ther. Drug Carrier Syst., 12, 1-99, 1995). Similarly, the single emulsion evaporation method also can enhance drug encapsulation by suitably increasing the concentration of a biodegradable polymer (PLGA) dissolved in an organic solvent phase. Typically, microspheres prepared by double emulsion evaporation are more porous than those prepared by single emulsion evaporation, and thus have increased surface areas, leading to a relatively high initial release rate of a drug.
However, the single and double emulsion evaporation methods for preparing microspheres, like the phase separation method, have the following disadvantages: difficulty in the removal of an organic solvent used for dissolving a biodegradable polymer, difficulty in mass production procedures due to changes in solvent removal rate, allergic reactions to gelatin used for increasing the viscosity of a primary emulsion, the possibility of a drug becoming denatured and losing its activity due to strong shearing force applied for making small microspheres during primary emulsion preparation, limited drug encapsulation, and the like.
The spray drying method has also been used for preparing finely atomized particles. In this method, typically, a solution of a material to be dried, or a suspension or emulsion in which a biodegradable polymer and a drug are homogenously dissolved, is supplied to a nozzle, sprayed through the nozzle, and exposed to heated air to evaporate the solvent used. In particular, in the case of preparing sustained release microspheres, the drug release rates of prepared microspheres greatly depend on the composition or content of a biodegradable polymer, the type or content of an additive, the composition of a solvent, and the like. In addition to the above processing parameters, other parameters affecting the morphology, size or properties of microspheres may be employed to control the release rates of drugs, the parameters including the type of a spray nozzle through which a spray solution is sprayed (for example, a type that atomizes droplets using compressed air, a type that atomizes droplets using centrifugal force when a spray solution flows into a disc rotating at a high speed, a type that atomizes droplets using ultrasonic waves generated when a vibrator vibrates, etc.), supply rate of a spray solution, and temperature, supplied amount and supply rate of drying air. In addition, the spray drying method, unlike other methods for preparing sustained release microspheres, is advantageous in that it provides a continuous process, which facilitates microsphere production and thus conversion from small-scale to large-scale production.
Although the spray drying method has the advantage of permitting large-scale production of microspheres, it has disadvantages as follows. The solvent used is not sufficiently removed merely by spray drying. The residual solvent causes a problem in the stability of the biodegradable polymer upon long-term storage, leading to changes in drug release profiles of microspheres. Another disadvantage of this method is that since biodegradable polymers used for drug encapsulation are mostly non-hydrophilic, microspheres prepared are not suspended well and are thus difficult to accurately administer.
As described above, most conventional methods of preparing sustained release microspheres employ a toxic solvent, and have problems including residue of the toxic solvent used, the microsphere size not being suitable for injection, poor suspendability of microspheres, and difficult mass production.
The present inventors intended to provide a process for preparing bioactive drug-loaded biodegradable polymer microspheres, which are easy for mass production, by solving the aforementioned problems.